Found at your friendly and voluminous neighborhood Walmart are these cheap and thoroughly acceptable polarized sunglasses.
Branded Berkley, you'll find them in this cardboard display bin in the fishing gear section for all of $4.96. Pricier ones are individually packaged, although these seem made well enough with no visual distortions I can see.
http://fastercat.smugmug.com/Other/M...L/image-XL.jpg